The field of the invention is that of optical telecommunications and more particularly that of long-haul telecommunications. In the present situation of ever increasing bit rates on long-haul transmission networks, increasing the transmission channel bit rate is inevitable, because it reduces the overall size, and more importantly the cost, of the terminal equipment. Accordingly, the next few years should see the deployment on the transport networks of telecommunication operators of the first wavelength division multiplexing (WDM) plant operating at 40 Gbit/s per wavelength and, in the longer term, at 160 Gbit/s per wavelength. This being the case, the requirements of transport networks in terms of optical time division multiplexing/demultiplexing (OTDM) will also increase. This being so, it is particularly beneficial to provide an all-optical WDM/OTDM conversion function, in order to be able to transfer the information conveyed by a plurality of wavelengths to a single carrier, and an all-optical OTDM/WDM conversion function, in order to be able to transfer to a plurality of optical carriers the information contained in an optical channel operating at a very high bit rate, typically at 40 Gbit/s, 160 Gbit/s or 640 Gbit/s. In the latter case, the number of optical carriers involved in the conversion process is equal to the number of OTDM components of the optical signal to be converted. These OTDM components may have a bit rate of 40 Gbit/s or 10 Gbit/s.
Solutions for providing this kind of WDM/OTDM and OTDM/WDM conversion exist already, and include all-electronic solutions that use opto-electronic transponders equipped with photo receivers or laser diodes to effect optical/electronic and electronic/optical conversion. Electronic components then handle time division multiplexing/demultiplexing. Those solutions are complex to implement, however, because they require two-fold optical/electronic and/or electronic/optical conversions and use a large number of components, which makes them difficult to install in the network because of obvious overall size problems. They are also limited in terms of electrical bandwidth. The major drawback of those solutions is that their bit rate is limited because the electronics used are not able to operate at bit rates of 40 Gbit/s and above.
There also exist all-optical solutions in which OTDM/WDM conversion consists in optical time division demultiplexing followed by wavelength conversion. Optical time division demultiplexing uses crossed phase modulation in a fiber, for example. That technology is very complex to implement, however. The optical time division demultiplexing may also be effected by means of non-linear optical mirrors using Mach-Zehnder, Michelson, or Sagnac interferometers. However, non-linear optical mirrors have the drawback of being unstable; their stability is in fact temperature-dependent. Wavelength conversion is effected by semiconductor optical amplifiers (SOA). A laser behind the SOA supplies the wavelength to which the signal must be converted. However, that solution uses a number of SOA and lasers equal to the number of wavelength conversions to be effected, with the result that the cost of that solution is very high, which rules out large-scale deployment in networks that are currently in full expansion. Moreover, SOAs are not completely bit rate transparent and distortion affecting the signal may occur.
WDM/OTDM conversion consists in converting the wavelength of each WDM signal to a single wavelength and then carrying out optical time division multiplexing. Wavelength conversion again necessitates the use of a number of SOA and lasers equal to the number of WDM signals, with the result that the cost of that solution is very high.
Finally, although the solutions described above for the two types of conversion (OTDM/WDM and WDM/OTDM) have the advantage of being all-optical solutions, which simplifies the signal processing system, they are able to operate only at low bit rates, below 40 Gbit/s.
Because of their limitations, the existing solutions cannot be used for WDM/OTDM or OTDM/WDM signal conversion at very high bit rates, that is to say bit rates above 40 Gbit/s.